It's a single focal length lens. Actually there are variable focal length primes but they're made for 35mm movie cameras. "Vari-primes" by zeiss come in a couple sizes and are much smaller than full range zooms but slightly larger than standard prime lenses of similar focal lengths.

The phrase "prime lens" is generally used to mean "lens with fixed focal length" today.

Long before zoom lenses came around, it used to mean the primary lens (set of lens elements or lenses, to be precise) in a complex optical system, in the sense that the system was designed for optimal performance with that set. You can add "supplementary lens" elements to the "prime lens" to change the focal length of the prime to get more mileage out of it. Supplementary lenses are calibrated in diopters; negative diopters increase the focal length of the resulting system; positive diopters reduce it.

In a sense, you could think of the 240mm lens in a 240mm/420mm convertible as the "prime", which can then be converted not by adding, but by removing a lens from the system.

With the advent of zoom lenses with their ability to change focal length continuously through the zoom range, "prime lens" lost its original meaning, as focal lengths change not by removing or adding elements (a discrete process) but by changing the distance between lenses (a continuous process), and came to mean "lens with fixed focal length".

"Prime lenses" have nothing to do with "prime numbers" :-)

Cheers,

- Phong

Last edited by Phong; 12-05-2005 at 03:36 PM. Click to view previous post history.